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.Dd December 15, 2015
.Dt SETUID 2
.Os
.Sh NAME
.Nm setuid ,
.Nm seteuid ,
.Nm setgid ,
.Nm setegid
.Nd set user and group ID
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn setuid "uid_t uid"
.Ft int
.Fn seteuid "uid_t euid"
.Ft int
.Fn setgid "gid_t gid"
.Ft int
.Fn setegid "gid_t egid"
.Sh DESCRIPTION
The
.Fn setuid
system call
sets the real and effective
user IDs and the saved set-user-ID of the current process
to the specified value.
.\" Comment out next block for !_POSIX_SAVED_IDS
.\" The real user ID and the saved set-user-ID are changed only if the
.\" effective user ID is that of the super user.
.\" I.e.
.\" .Fn setuid
.\" system call is equal to
.\" .Fn seteuid
.\" system call if the effective user ID is not that of the super user.
.\" End of block
The
.Fn setuid
system call is permitted if the specified ID is equal to the real user ID
.\" Comment out next line for !_POSIX_SAVED_IDS
.\" or the saved set-user-ID
.\" Next line is for Appendix B.4.2.2 case.
or the effective user ID
of the process, or if the effective user ID is that of the super user.
.Pp
The
.Fn setgid
system call
sets the real and effective
group IDs and the saved set-group-ID of the current process
to the specified value.
.\" Comment out next block for !_POSIX_SAVED_IDS
.\" The real group ID and the saved set-group-ID are changed only if the
.\" effective user ID is that of the super user.
.\" I.e.
.\" .Fn setgid
.\" system call is equal to
.\" .Fn setegid
.\" system call if the effective user ID is not that of the super user.
.\" End of block
The
.Fn setgid
system call is permitted if the specified ID is equal to the real group ID
.\" Comment out next line for !_POSIX_SAVED_IDS
.\" or the saved set-group-ID
.\" Next line is for Appendix B.4.2.2 case.
or the effective group ID
of the process, or if the effective user ID is that of the super user.
.Pp
The
.Fn seteuid
system call
.Pq Fn setegid
sets the effective user ID (group ID) of the
current process.
The effective user ID may be set to the value
of the real user ID or the saved set-user-ID (see
.Xr intro 2
and
.Xr execve 2 ) ;
in this way, the effective user ID of a set-user-ID executable
may be toggled by switching to the real user ID, then re-enabled
by reverting to the set-user-ID value.
Similarly, the effective group ID may be set to the value
of the real group ID or the saved set-group-ID.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The system calls will fail if:
.Bl -tag -width Er
.It Bq Er EPERM
The user is not the super user and the ID
specified is not the real, effective ID, or saved ID.
.El
.Sh SEE ALSO
.Xr getgid 2 ,
.Xr getuid 2 ,
.Xr issetugid 2 ,
.Xr setregid 2 ,
.Xr setreuid 2
.Sh STANDARDS
The
.Fn setuid
and
.Fn setgid
system calls are compliant with the
.St -p1003.1-90
specification with
.Li _POSIX_SAVED_IDS
.\" Uncomment next line for !_POSIX_SAVED_IDS
not
defined with the permitted extensions from Appendix B.4.2.2.
The
.Fn seteuid
and
.Fn setegid
system calls are extensions based on the
.Tn POSIX
concept of
.Li _POSIX_SAVED_IDS ,
and have been proposed for a future revision of the standard.
.Sh HISTORY
The
.Fn setuid
function appeared in
.At v1 .
The
.Fn setgid
function appeared in
.At v4 .
.Sh SECURITY CONSIDERATIONS
Read and write permissions to files are determined upon a call to
.Xr open 2 .
Once a file descriptor is open, dropping privilege does not affect
the process's read/write permissions, even if the user ID specified
has no read or write permissions to the file.
These files normally remain open in any new process executed,
resulting in a user being able to read or modify
potentially sensitive data.
.Pp
To prevent these files from remaining open after an
.Xr exec 3
call, be sure to set the close-on-exec flag:
.Bd -literal
void
pseudocode(void)
{
	int fd;
	/* ... */

	fd = open("/path/to/sensitive/data", O_RDWR | O_CLOEXEC);
	if (fd == -1)
		err(1, "open");

	/* ... */
	execve(path, argv, environ);
}
.Ed
